For most radiation detectors the detectivity increases with decreasing detector area. Conventional infra-red optical systems use reflecting or refracting elements made from special infrared optical materials with broad spectral transmission. There are practical limits to the optical gain which can be achieved using such optical systems, as such systems can be difficult and expensive to make in large sizes, and the equipment in which they are mounted becomes increasingly bulky. A practical approach which has been taken is to achieve optical gain directly at the detector using relatively small optical elements. This has been achieved by the immersed detector in which the infrared window, generally in the form of a lens, is in optical contact with the infrared detector. The lens, referred to as an immersion lens, may be of any suitable material, such as fused aluminum oxide, a type of dielectric material, or a semiconductor material such as germanium or silicon. The type of lens material used with depend on the particular application, and generally where increased responsivity is required, semiconductor lenses, such as germanium, which has a refractive index of 4, will be used.
In adhering the infrared detector to the infrared window to provide the optical contact between the lens and the infrared detector, there is provided a very thin layer of insulating material which is generally glass-like, such as selenium modified with arsenic glass. Other types of immersion material may be utilized in accordance with a given application. The immersion material performs the additional function of providing a path of moderate thermal impedance between the infrared detector and the lens which in many cases acts as a heat sink for the detector. In thermistor bolometers, if a semiconductor window or lens is utilized, the immersion material acts as an insulator for preventing the shorting out of the thermistor flake. Immersing the infrared detector, such as the thermistor flake, on the lens involves an extremely delicate operation which is generally carried out under a microscope. For the thermistor bolometer, for example, sheets of pastes of oxide are made up, cut into tiny flakes of the desired size, and then sintered and annealed. The flakes are very thin, for example on the order of 10 microns or less, and are very fragile. So that electrical connections can be made to the flake, it is necessary to deposit a conductor such as gold on a masked thermistor flake to produce lead areas to which leads such as platinum wires may be attached. An area on the back of the window or immersion lens where the flakes are to be mounted is coated with a thin film of immersion glass, for example 10 microns, and the layer is generally applied thereto by vacuum deposition. Each thermistor is then placed on the immersion glass layer, with a weight placed thereon, and the flake assembly centered on an optical mechanical axis of the lens. The preimmersion assembly is placed in an oven and the temperature raised to the softening point of the immersion glass wherein the weights on the thermistor flake cause it slowly to sink into the softened glass until spacing from the back of the lens has reached the desired value, whereupon the heat is turned off and the bolometer is cooled slowly. This is a delicate operation and requires continuous observation by highly skilled personnel. When the operation is finished, a very delicate detector is produced with lead wires connected thereto which must be connected to bases or external circuitry with great care.
Accordingly, it is an object of the present invention to provide an improved construction for an immersed type infrared detector which is simple to fabricate and far more rugged than the prior art type of immersed detectors.
A further object of this invention is to provide a new and improved construction for an immersed type infrared detector which can withstand vibration, shock, and high acceleration levels without damage thereto.